In the past, a control system of an internal combustion engine which is provided with an air-fuel ratio sensor in an exhaust passage of the internal combustion engine and controls the amount of fuel fed to the internal combustion engine based on the output of this air-fuel ratio sensor, has been widely known (for example, see PLTs 1 to 6). Further, the air-fuel ratio sensor which is used in such a control system has also been widely known.
Such air-fuel ratio sensors may be roughly divided into single-cell types of air-fuel ratio sensors (for example, PLTs 2 and 4) and double-cell types of air-fuel ratio sensors (for example, PLTs 1, 3, and 5). In a single-cell type of air-fuel ratio sensor, only a single cell comprised of a solid electrolyte layer which can pass oxygen ions and two electrodes which are provided on both side surfaces of the layer, is provided. One of the electrodes thereof is exposed to the atmosphere, while the other electrode is exposed to the exhaust gas through a diffusion regulating layer. In the thus configured single-cell type of air-fuel ratio sensor, voltage is applied across two electrodes which are arranged on the both side surfaces of the solid electrolyte layer. Along with this, between the two side surfaces of the solid electrolyte layer, movement of oxygen ions occurs in accordance with the ratio of concentration of oxygen between these side surfaces. By detecting the current generated by this movement of oxygen ions, the air-fuel ratio of the exhaust gas (below, also referred to as the “exhaust air-fuel ratio”) is detected (for example, PLT 2).
On the other hand, in a double-cell type of air-fuel ratio sensor, two cells, each comprised of a solid electrolyte layer which can pass oxygen ions and two electrodes which are provided on both side surfaces of the layer, are provided. One cell (reference cell) among these is configured so that the detected voltage (electromotive force) changes in accordance with a concentration of oxygen in exhaust gas in a measured gas chamber. Further, the other cell (pump cell) pumps oxygen in and pumps it out with respect to the exhaust gas in the measured gas chamber, in accordance with a pump current. In particular, the pump current of the pump cell is set so as to pump in oxygen and pump it out so as to make the detected voltage which is detected at the reference cell conform to a target voltage value. By detecting this pump current, the exhaust air-fuel ratio is detected.